Drilling techniques for producing wellbores to great depths in the earth are well known and are widely used, especially in the exploration for and production of hydrocarbons. These wells are typically produced by the use of a drill bit positioned on the lower end of a drill string which is supported for rotation to cause the bit to drill into the earth with the drilling being stopped periodically, with the drill string being lifted and supported on slips or similar devices so that a new section of pipe can be attached to the top drill pipe section. These drill pipe sections are fitted with upset ends so that they can be threaded with male fittings on one end and female fittings on the other end. These drill pipe sections are typically about 30 feet long and when joined together can be used to drill for great distances into the earth.
In drilling such boreholes into the earth, it is not uncommon to case the upper portions of the well after it has been drilled to a suitable depth. Frequently the diameter of the wellbore is decreased as it is drilled deeper into the earth. These techniques are well known to those skilled in the art.
During drilling a drill string is positioned from a surface into the wellbore and to the bottom of the wellbore so that the bit can be rotated. The bit is typically rotated by passing a drilling fluid downwardly through the drill pipe to drive the drill bit and extend the bottom of the hole downwardly.
Drilling fluids are well known and comprise water-based drilling fluid and oil-based drilling fluid. Further specialized drilling fluids, such as drill-in fluids may also be used. The drilling fluids are typically made up to have a specific gravity so that a column of drilling fluid of a height equal to the wellbore depth exerts a bottom hole pressure equal to the anticipated pressure in the formations penetrated by the wellbore over the entire depth of the well. This drilling fluid pressure tends to inhibit the production of gases and oil formation fluids into the wellbore or to the surface when greater than the formation pressure. It also inhibits events such as kicks and blow-outs where high pressure permeable formations are encountered. The industry has developed numerous techniques for detecting such kicks and blow-outs early to prevent significant damage to the drilling apparatus and to prevent blowing the entire mud column out of the wellbore and possibly contaminating the surrounding area with hydrocarbons.
One technique for identifying such high-pressure formations is shown in U.S. Pat. No. 5,214,251 issued May 5, 1993, to Orban, et al (the '251 Patent) and assigned to Schlumberger Technology Corporation. A second closely related patent is U.S. Pat. No. 5,354,956 issued Oct. 11, 1994 to Orban, et al (the '956 Patent) and assigned to Schlumberger Technology Corporation. Both these patents are hereby incorporated in their entirety by reference. These references disclose methods for detecting large gas bubbles which may be discharged into the wellbore from a high-pressure formation (kicks) and possibly damage the well and blow all the drilling fluid from the well onto the earth surface.
It is highly desirable that such conditions be identified prior to drilling into such high-pressure formations so that the weight of the drilling fluid can be adjusted to prevent the blow-out.
Accordingly, considerable effort has been directed to the development of methods for detecting subtle amounts of gas invading a wellbore as drilling is conducted. It is recognized that it would be desirable to know the pressure of small amounts of gas in the drilling fluid. Many wells are drilled slightly under-balanced. In other words, the drilling fluid is pumped into the drill pipe at a pressure such that the drilling fluid passing through the drill and into the annulus between the outside of the drill pipe and the inside of the borehole is at a pressure slightly less than that anticipated from the formations through which the well passes. This permits the drilling of the well without unduly contaminating the faces and near-wellbore portions of the formations penetrated by the well. Use of over-pressure drilling can force drilling fluid into the formations penetrated by the wellbore. Drilling fluid components in the well formation faces and near-wellbore portions of the formation can be detrimental to the production of fluids from the formation after the well has been completed.
In other instances, the well may be drilled slightly over-balanced but the drilling fluid may have a weight insufficient to maintain over-balance on the well if the pumps are stopped. This is also an under-balanced condition when the pumps are off. Such conditions exist periodically during the drilling operation because it is periodically necessary to stop the pumps, disconnect from the drill pipe and add a new section of drill pipe to allow the drilling to proceed to an even greater depth. The pressure resulting from the weight of the column of the drilling fluid is referred to as a hydrostatic pressure. This hydrostatic pressure also can be greater than or less than the pressure in the formation. Desirably this hydrostatic pressure is to be slightly greater than the pressure in the formations penetrated by the wellbore for a safety perspective. The desire of this invention is to detect the condition of the hydrostatic pressure being slightly less than the pressure in the formations penetrated by the well when these conditions are first observed in the pumps off condition when the hydrostatic pressure in the well is slightly less than in the pumps on condition.
Of course if an over-balance, i.e., a pressure greater than the pressure in the pores of the formations penetrated by the wellbore is used then little, if any, gas will enter the wellbore from the formations during drilling. There may be gas associated with the formation that has been excavated by the bit that is released as the formation cuttings are returned to the surface but the amount of gas present will then be independent of the pumps-on/pumps-off condition. When an over-balanced condition exits, portions of the drilling fluid will enter the formations and constitute an obstacle to the production of fluids from the formations.
In a preferred embodiment the hydrostatic pressure in the well during pumping of the drilling fluid is slightly over-balanced relative to the formation pressure with the hydrostatic pressure being slightly less when the pumps are off. In such instances very small amounts of formation gas can enter the wellbore from low permeability formations, such as shale. This gas may exist as a free fluid in the formation or it may be dissolved in water. The presence of this small amount of gas entering the wellbore is indicative that a higher-pressure formation may be exposed in the wellbore. As a result, it is desirable to check this gas periodically to determine whether the amount of gas entering the well under comparable conditions is increasing or stable when pumps are turned on and off.
The most commonly used methods of making this determination is to separate the gas from the drilling fluid at the surface. This is an effective method for determining how much gas may be in the drilling fluid but unfortunately in a well of any substantial depth it may take two to three hours for this drilling fluid to reach the earth surface. This may be too late to avoid drilling into a high-pressure formation without making adequate preparations.
Accordingly an improved method has been sought for determining the amount of gas in the drilling fluid at a given time without the long wait for the drilling fluid to move to the earth surface.